As described In U.S. Pat. No. 2,559,752 to Berry, fluorosurfactants are used as a polymerization aid in the dispersion polymerization of fluoropolymers functioning as a non-telogenic dispersing agent. As has been further taught in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls), these expensive fluorosurfactants can be recovered either from the aqueous phase after the polymer has been coagulated from the dispersion or in the aqueous polymer dispersions prior to concentration. A preferred method of recovering the fluorosurfactant from the fluoropolymer dispersion as taught in both Kuhls and Seki et al. is by adsorption onto an ion exchange resin.
Fluoropolymer manufacturing processes that involve ion exchange adsorption as an added process step suffer from several disadvantages. For example, a process employing the addition of ion exchange resin to the dispersion in a stirred tank, typically in the form of resin beads, requires long contact times with stirring to effect the adsorption of the fluorosurfactant. In practical terms, the rate of adsorption is limited by the rate and efficacy of the stirring; the amount, bead size, and condition of the ion exchange resin; the relative chemical potentials of the particular ion exchange resin being used and anions to be exchanged; and the temperature. Because of such limitations, the recovery of the fluorosurfactant from the fluoropolymer dispersion using ion exchange resin as disclosed in the prior art takes a number of hours, i.e., typical treatment times longer than a typical polymerization cycle. Thus, in order to avoid increasing the length of the total manufacturing cycle time, either multiple tanks or a tank large enough to receive several polymerization batches must be used and the ion exchange step must be run with high production efficiency.
Another disadvantage of the prior art method is that the shearing forces associated long stirring times has the propensity to cause some agglomeration of the primary polymerization particles forming large particles resulting in poorer dispersion stability due to particle settling. Further, the amount of ion exchange resin necessary with the prior art method to keep the time of the ion exchange adsorption short so as to keep up with batch polymerization cycles can require about 3 times the amount necessary to achieve the same level of fluorosurfactant reduction that could be achieved if longer contact times were available.
If the dispersion is passed through a fixed bed of ion exchange instead of using stirred resin beads, the rate of exchange is also slow, limited by the necessity of the dispersion to pass through the bed at a sufficiently slow rate to provide adequate fluorosurfactant adsorption. The slow passage through the ion exchange bed leads to the first portion of the dispersion having all of the fluorosurfactant removed. Later portions have lesser amounts removed as the top of the bed becomes progressively more exhausted. The final portion may have little of the fluorosurfactant removed as the bed reaches exhaustion. Care must be exercised in later blending so that the final product does not vary as a result of differences in bed exhaustion. Moreover, much labor or capital for mechanical equipment must be employed to replenish or replace the ion exchange resin in the vessel containing the fixed bed. The overall rate compares with the stirred bead technique.
What is desired is an efficient manufacturing process for fluoropolymer dispersion which recovers fluorosurfactant directly from dispersion without increasing the dispersion production cycle time, avoids the need for larger scale production equipment, and provides high quality, uniform product without shearing damage and particle agglomeration.